In any wireless communication system, a signal transmitted from one signaling part to another is distorted due to dynamic properties of a radio channel through which it is transmitted. The radio channel between a transmitter and a receiver can be characterized by channel state information describing how the signal propagates from the transmitter to the receiver. The channel state information is estimated at the receiver and conveyed to the transmitter, and used for compensating e.g. for scattering, fading and power level reduction related to distance between the transmitter and receiver.
In a time division duplex (TDD) based communication system, reciprocity between downlink (DL, signaling from base station to user equipment) and uplink (UL, signaling from user equipment to base station) can often be assumed. The channel state information of the user equipment (UE) and the base station may be assumed to be equal, which provides a large advantage in highly reduced signaling and thereby increased system performance.
However, the assumption that the radio channel is reciprocal is not always true. The channel reciprocity is, for example, lost when a conventional one-way repeater is introduced in the communication system. Such repeaters are often desirable in the wireless communication system for overcoming e.g. power level reductions resulting e.g. from large distances. However, due to non-coherent radio paths, used for DL and UL, respectively, there might be both amplitude and phase differences between the two links, introduced by the repeater. The differences can severely affect performance.
For example, in a system in which channel weights in DL, based on UL signals, are designed for constructively adding signal components from different signal paths at the receiving end, the introduction of a repeater may destroy the reciprocity. In particular, signal paths from the base station may be received directly by the user equipment, as well as via the repeater. Amplitude and phase errors result from the false assumption that the channel is reciprocal and that the same channel state information can be used in uplink and downlink. Due to these amplitude and phase errors, the signal components of the different paths may add up less efficiently or even destructively. In short, the repeater introduces phase differences as well as amplitude differences, which is highly detrimental for the system performance.
From the above it is clear that it would be advantageous to be able to benefit from the reciprocity of communication channels to as a large extent as possible, even when a repeater is utilized.